A manufacturer or a user of bulk materials generally wants to keep track of the amount of goods contained in a storage vessel, such as a large bin or silo. A number of conventional instruments mounted inside the storage container are available for this purpose. There are level measuring indicators, some of which are mechanical, such as so called "plumb bobs", and others of which are electric, such as capacity probes, ultrasound ranging systems, and microwave ranging systems.
However, none of these conventional instruments give very accurate information about the weight of the stored material, and, since they are located inside the storage container, they can easily corrode or gum up with sticky material and require servicing. As a result of these problems with internal instruments, the presently preferred approach is to weigh the contents of the silo. This approach gives much more precise and valuable information, and, since the weighing devices are located outside the tank, the instruments are not subject to the effect of corrosive or abrasive materials, and thus normally do not require servicing.
One conventional device using load cells to weigh the contents of a storage container is described in U.S. Pat. No. 4,596,155 to Kistler. The Kistler patent describes a load cell which basically consists of an inverted cup inside of which a two axis strain sensor is mounted. When a load is applied to the center of the top surface of this load cell, this top surface is spherically deformed. The magnitude of this deformation is measured using the two-axis strain sensor to provide an accurate measure of the applied load. The basic simplicity, relatively low cost and good accuracy of this type of load cell has made it very successful. However, like most other load cells, it cannot be bolted to either the foundation or a floor or other support structure. As a result, the vessel simply rests on the load cells without being solidly tied down. Consequently, a tall outdoor silo could easily topple in strong winds or during a minor earthquake. To prevent this, horizontal flexible tie rods are used to laterally restrain the silo or tank. Vertical tie rods, which cannot contact the tank or its support structure, are installed to prevent it from toppling over. This makes installation rather complex and expensive and large measuring errors may result if, for example, the tank is vertically restrained as a result of improper installation of the vertical tie bolts.
Several efforts have been made to modify the device shown in the Kistler patent to provide attachment to a floor or support structure through bolting or welding. However, all of these approaches either degrade the accuracy of the load cell or make the load cell incapable of withstanding certain types of loads without being damaged. For example, U.S. Pat. No. 4,166,997 to Kistler describes a cylindrical load cell having a lower mounting plate that is secured by multiple bolts to a floor, and an upper mounting plate that is secured by multiple bolts to a storage container support member. The load cell has a circular flex plate that is coupled to the upper and lower mounting plate at different radial positions so that the plate deforms in proportion to the weight of the storage container and its contents. The magnitude of the flex plate strain is measured by conventional strain sensing means to provide an indication of the weight of the contents of the storage container.
Although the device disclosed in the Kistler '997 patent may perform well in some applications, it is easily damaged by bending moments applied to the load cell. These bending moments can be caused by angular misalignments of the storage vessel support structures that are secured to the load cell. More particularly, since the load cell disclosed in the Kistler '997 patent is bolted to a vessel support member at multiple points, the angle between the support member and the upper surface of the load cell is fixed. Similarly, since the load cell disclosed in the Kistler '997 patent is bolted to a floor surface at multiple points, the angle between the floor surface and the lower surface of the load cell is fixed. As a result, any change in the angle between the support member and the floor surface must be accommodated by a lateral bending of the load cell.
Bending moments can also be applied to the load cell described in the '997 Kistler patent by lateral forces that the support structure applies to the load cell. A lateral force applied to the load cell by the support member will impart a bending moment to the load cell having a magnitude that is equal to the product of the force and the vertical thickness of the load cell (ie., the product of the force and the moment arm).
The design of the load cell described in the Kistler '997 patent is inherently incapable of accommodating substantial bending moments caused by either angular misalignments or lateral forces as described above. As a result, the load cells described in the Kistler '997 patent have been subject to frequent damage.